A problem on the subject of recovery of petroleum is the monitoring of the progress or frontal advance of pressurized and/or heated zones during the injection of steam, air, water, or other fluids into oil bearing or bituminous reservoirs. The recovery of heavy oils or bitumen is desired particularly from large, shallow deposits such as one of the Athabasca oil sand deposits in Alberta, Canada. The availability of a simple means to locate the advancing front of injected fluids would aid in the evaluation and improvement of experimental recovery processes and would assist optimizing commercial applications.